In Long Term Evolution (LTE) and a succeeding system of LTE (for example, also referred to as LTE-Advanced (LTE-A), Future Radio Access (FRA), or 4G), device-to-device (D2D) technologies for enabling user terminals to perform direct communication without involving radio base stations have been examined (for example, Non-Patent Document 1).
D2D enables user apparatuses to communicate with each other even when traffic between the user apparatuses and base stations is reduced or communication with base stations is disabled in disasters.
D2D is broadly classified into D2D discovery (also referred to as D2D finding) for discovering other user terminals with which communication is enabled and D2D communication (also referred to as D2D direct communication, D2D communication, or inter-terminal direct communication) for enabling terminals to perform direct communication. Hereinafter, when the D2D communication and the D2D discovery are not particularly distinguished from each other, the D2D communication and the D2D discovery are simply referred to as D2D. Signals transmitted and received by D2D are referred to as D2D signals.
In 3rd Generation Partnership Project (3GPP), V2X has been examined to be realized by expanding D2D functions. Here, V2X is a part of intelligent transport systems (ITS) and is a general term for vehicle to vehicle (V2V) meaning a communication form performed between automobiles, vehicle to infrastructure (V2I) meaning a communication form performed between an automobile and a road-side unit (RSU) installed on a road side, vehicle to nomadic device (V2N) meaning a communication form performed between an automobile and a mobile terminal of a driver, and vehicle to pedestrian (V2P) meaning a communication form performed between an automobile and a mobile terminal of a pedestrian, as illustrated in FIG. 1.